I had a chance to run my 58 today. It is set up with a stock muffler, which is the same as the 115, but with the exhaust flange moved. The engine came very rich so I had to lean it quite a bit. When all was said and done it would turn a menz-s 23x8 at 7200 rpm. Total run time was about 10 minutes. The engine does not seem to vibrate at idle very much, and throttle response is linear and smooth.
Now I have to move my batteries from behind my canopy to the motor box.